P-xylene (PX) is a key organic chemical raw material mainly employed in preparing important organic intermediates, such as terephthalic acid, etc., for producing polymers. The p-xylene has relatively abundant sources and a low price, which can be obtained by methods such as disproportionation of toluene, isomerization of xylene, aromatization of methanol, and reaction of 2,5-dimethylfuran with ethylene, etc. At present, a productive capacity of p-xylene is about 15 million tons/year in China. Therefore, it will greatly promote the development of p-xylene industry by expanding a development and utilization of downstream products of the p-xylene.
With the continuous recognition and broad application of oxygen-containing aromatic compounds, oxygen-containing organic compounds, such as terephthalic acid, 2,5-dihydroxyterephthalic acid, 4-(Hydroxymethyl)benzoic acid, etc., play more and more prominent roles in human production and living. These organic compounds are mainly used for synthesizing polymers, novel lithium batteries, drugs, etc. Although these oxides are widely used, the oxides such as 2,5-dihydroxyterephthalic acid, 4-(Hydroxymethyl)benzoic acid, etc. are rarely synthesized by a one-step oxidation using p-xylene which has a low price and abundant sources as a raw material, except that the terephthalic acid is obtained by an oxidation of the p-xylene.
In recent years, 4-(Hydroxymethyl)benzoic acid has been widely used in the synthesis and application of drugs, pharmaceutical intermediates, photocatalytic materials and optoelectronic materials (The Journal of organic chemistry, 2012, 78(1): 83-92; Angewandte Chemie International Edition, 2011, 50(33): 7599-7603; Crystal Growth & Design, 2011, 11(5): 1502-1511; Chemical Communications, 2012, 48(10): 1526-1528; Angewandte Chemie International Edition, 2012, 51(27): 6631-6634), and 4-(Hydroxymethyl)benzoic acid is considered to be an extremely high-value chemical intermediate.
Although the application of 4-(Hydroxymethyl)benzoic acid has attracted widespread attention, synthesis methods of 4-(Hydroxymethyl)benzoic acid have no big breakthrough. Conventional synthesis methods are as follows. 4-(Hydroxymethyl)benzoic acid is obtained by reacting 4-methylbenzoic acid with bromine to form 4-bromomethylbenzoic acid, and then hydrolyzing the 4-bromomethylbenzoic acid (FIG. 1, A). Alternatively, 4-(Hydroxymethyl)benzoic acid is obtained by selectively oxidizing 1,4-benzenedimethanol using a catalyst (FIG. 1, B) (ChemCatChem, 2010, 2(10): 1286-1295; U.S. Pat. No. 9,328,050B1). For the above two methods, many reaction steps are needed. It is necessary to first obtain 4-methylbenzoic acid or 1,4-dicarboxybenzene by methods such as oxidizing p-xylene, etc., and then next reaction can be performed, which causes a lot of by-products. Moreover, bromides produced in the reaction cause corrosion to devices or expensive catalysts are used in the above methods, which deviates from the concept of green chemistry. Therefore, a more convenient, efficient, and inexpensive synthesis method is needed for producing large amounts of 4-(Hydroxymethyl)benzoic acid.
In view of the above problems, it is quite necessary to develop a milder and greener method for synthesizing 4-(Hydroxymethyl)benzoic acid. M. G. Bramucci et al. obtain 4-(Hydroxymethyl)benzoic acid by using a xylene monooxygenase as a catalyst to perform a biomimetic catalytic oxidation on p-xylene (U.S. Pat. No. 20030073206A1). However, this method has problems such as long reaction time, low yield, and complicated cultivation of biocatalyst.